1. Field of the Invention
The present invention relates to a spread spectrum process and apparatus for code division multiple access communication, and, more particularly, to processes and apparatus for maintaining orthogonality between channel signals using zero-crossing detection.
2. Description of the Related Art
Code division multiple access communication systems are generally discussed in such exemplars of the art as, for example, the System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System, U.S. Pat. No. 5,103,459 issued to Klein S . Gilhousen, et al. Other background may be found in Spread Spectrum Communications by Marvin K. Simon, et al., published by Computer Science Press, 1989; Spread Spectrum Communications Handbook, by Marvin K. Simon, et al., published by McGraw-Hill in 1994; and Spread Spectrum System With Commercial Applications, by Robert C. Dixon, published by John Wiley and Sons in 1994. Other efforts include the Data Recovery Technique for Asynchronous CDMA Systems, U.S. Pat. No. 5,431,395 issued to Bi; All Digital Maximum Likelihood Based Spread Spectrum Receiver, U.S. Pat. No. 5,361,276 issued to Subramanian; Spread Spectrum Communication System And An Apparatus For Communication Utilizing This System, U.S. Pat. No. 5,400,359, issued to Hikoso, et al.; and Method And Apparatus For Bifurcating Signal Transmission Over In-Phase And Quadrature Phase Spread Spectrum Communication Channels, U.S. Pat. No. 5,414,728, issued to Zehavi. More recent effort include the System And Method For Generating Signal Waveforms In a CDMA Cellular Telephone System, U.S. Pat. No. 5,416,797 issued to Gilhousen, et al.; Methods Of And Devices for Enhancing Communications That Use Spread Spectrum Technology By Using Variable Code Techniques, U.S. Pat. No. 5,546,420 issued to Seshadri et al.; the Direct Sequence Code Division Multiple Access (DS-CDMA) Communication System And A Receiver For Use In Such A System, U.S. Pat. No. 5,550,810 issued to Monogioudis, et al.; Synchronous CDMA Transmitter/Receiver, U.S. Pat. No. 5,583,835 issued to Giallorenzi, et al.; Standalone Canceller Of Narrow Band Interference For Spread Spectrum Receiver, U.S. Pat. No. 5,596,600 issued to Dimos, et al.; the DS/CDMA Receiver For High-Speed Fading Environment, U.S. Pat. No. 5,646,964 issued to Ushirokawa, et al.; CDMA Demodulator And Demodulation Method, U.S. Pat. No. 5,694,388 issued to Sawahashi, et al.; and CDMA Communication System In Which Interference Removing Capability Is Improved, U.S. Pat. No. 5,734,647 issued to Yoshida, et al.
A communication system that uses direct sequence spread spectrum is commonly known as a direct sequence code division multiple access (DS/CDMA) system, in accordance with TIA/EIA standard IS-95. Individual users of the system use the same radio frequency (RF), but are separated by the use of individual spread codes. Exemplars of data transmission and reception in DS/CDMA systems may be found in Korean patent application No. 1994/20801 for a Data Transceiver In Spread Spectrum Communication System Using Pilot Channel; Korean patent application No. 1994/30497 for a Data Transceiver In Spread Spectrum Multiple Access Communication Using Pilot Channel; Synchronous Transmitter And Receiver Of Spread Spectrum Communication Method, U.S. Pat. No. 5,675,608 issued to Kim, et al.; and the Data Transmitter And Receiver Of A Spread Spectrum Communication System Using A Pilot Channel, U.S. Pat. No. 5,712,869 issued to Lee, et al.
One approach to spread spectrum communication contemplates offset quadrature phase shift keying (i.e., xe2x80x9cOQPSKxe2x80x9d) direct sequence code division multiple access (i.e., xe2x80x9cDS/CDMAxe2x80x9d communication system. Typically, I-channel (or I-arm) input data is multiplied by an orthogonal code WI(t) to orthogonally modulate the I-channel input data DI(t), and Q-channel (or Q-arm) input data is multiplied by an orthogonal code in order to orthogonally modulate the Q-channel input data. The orthogonally modulated I- and Q-channel signals are gain controlled in gain controllers and then applied to a spectrum spreader constructed of a plurality of multipliers that multiply output from the gain controllers by an I-channel spreading sequence and a Q-channel spreading sequence. Adders and subtractors combine the output from the multipliers in order to generate an I-channel spread signal and a Q-channel spread signal. For example, a spreader may generates a difference between the signals output from the multipliers as an I-channel signal, and a sum of the signals output from the multipliers as the Q-channel signal.
Conventional spread spectrum circuits delay the Q-channel spread signal by one-half chip to prevent zero-crossing of the spread signals, in an effort to avoid zero-crossing of the transmission signals. It is generally believed that by avoiding zero-crossing, finite impulse response (i.e., FIR) filtered signals have a reduced regrowth of sidelobes after amplification by a non-linear circuit such as a power amplifier at a subsequent stage.
Code division multiple access systems modulate the user channel using the orthogonal code. The time and phase of the one channel signal should coincide with that of the other channel signal in order to maintain the orthogonality between the two channel signals. We have found, however, that unlike quadrature phase shift keyed, direct sequence code division multiple access (QPSK DS/CDMA) systems, an offset quadrature phase shift keyed, direct sequence code division multiple access (OQPSK DS/CDMA) system cannot maintain orthogonality between the I-channel signal and the Q-channel signal; we have found that this results in the occurrence of unacceptable phase error. That is, when an output signal of an offset quadrature phase shift keyed, direct sequence, code division multiple access system is demodulated at the receiver, the orthogonality between the I-channel signal and Q-channel signal cannot be maintained accurately even in the absence of channel noises. We have noticed that this inability to maintain orthogonality between the I-channel signal and the Q-channel signal causes phase error due to interference between the channels; this results in degradation of the performance of the system.
It is, therefore, an object of the present invention to provide an improved spread spectrum communication process and circuit.
It is still another object to provide a spread spectrum apparatus and method capable of maintaining orthogonality between transmission signals in a code division multiple access communication system.
It is yet another object to provide processes and apparatus enabling code division multiple access communication in spread spectrum communications.
It is still yet another object to provide processes and circuits able to improve maintenance of orthogonality between channel signals by detecting zero-crossing during code division multiple access communication.
It is another object of the present invention to provide an apparatus and method capable of avoiding zero-crossing while maintaining an orthogonality by determining whether or not zero-crossing occurs, outputting spread spectrum signals as they are when zero-crossing does not occur and randomly delaying the spread spectrum signals when the zero-crossing occurs in a code division multiple access communication system.
These and other objects may be attained with a spread spectrum apparatus in a code division multiple access communication system. In the spread spectrum apparatus, a spreader combines first and second input signals with corresponding pseudo-random noise sequences to generate first and second spread signals. A zero-crossing detector determines whether zero-crossing occurs between the first spread signal and the second spread signal, to generate a zero-crossing detection signal. A first delay staggers the first spread signal in a first direction and a second delay staggers the second spread signal in a second direction. A first selector selects one of the first spread signal and a first staggered signal output from the first delay in response to the zero-crossing detection signal, and a second selector selects one of the second spread signal and a second staggered signal output from the second delay in response to the zero-crossing detection signal. Therefore, the first and second staggered signals are selected when the zero-crossing occurs, and the first and second spread signals are selected when the zero-crossing does not occur, thereby maintaining an orthogonality of transmission signals. Here, the first channel is an in-phase channel and the second channel is a quadrature phase channel.
Preferably, the first delay shifts the first spread signal in a positive direction by a preset chip and the second delay shifts the second spread signal in a negative direction by a preset chip. For example, the first delay shifts the first spread signal by +xc2xc chip and the second delay shifts the second spread signal by xe2x88x92xc2xc chip.